The invention pertains to a method for casting polymeric gels from volatile mixtures of monomers in open molds and to an apparatus for performing this method.
One of the most efficient methods of manufacturing shaped articles from plastics is the casting of monomers or their mixtures in molds under polymerization conditions. The method of casting in open molds can be used for some purposes in particular advantageously, for example, for casting foils on horizontal plates or for casting contact lenses in rotating open molds demarcated by a sharp edge. The latter method has been so far applicable only for such starting monomer mixtures where none of the components are pronouncedly volatile under the conditions of the entire procedure. However, if even only one of the monomers used, or a single component of the solution of monomer mixture employed, is volatile under the polymerization conditions, the more volatile components will evaporate from the open surface of the cast mixture and the composition of the surface layers will change. A casting obtained in this way is a substantially nonhomogeneous composition with the result that the casting is extensively deformed.
Thus, for example, contact lenses prepared by centrifugal casting of a monomer mixture of hydroxyethyl methacrylate and acrylic acid in the presence of a crosslinking agent or in the presence of inert solvents always perform more poorly due to the lower hydrophilicity of their inner surface caused by loss of the more hydrophilic component (i.e., acrylic acid), and such cast lenses always roll up in an uncontrollable way. Similarly, gels cast from a mixture of N-vinyl-pyrrolidone and methyl methacrylate lose the much more volatile methyl methacrylate component at the free surface. As a consequence, the upper layers are more swellable with water and the lenses are sometimes deformed to such an extent that they turn inside out in an uncontrollable way after swelling.
Similar difficulties also occur in the polymerization of substantially nonvolatile monomers which are diluted with volatile solvents. For example, if water is used as a solvent in the polymerization of mixtures of hydroxyethyl methacrylate with sodium methacrylate, the mixture becomes concentrated at its free surface by evaporation of water so that the resulting hydrophilic gel, brought into equilibrium with water, is more contracted on the originally open side than on the other side thereby causing its extensive deformation.
These difficulties also occur to a lesser extent when the polymerization of the monomer mixture is carried out with an open surface but in closed molds if this surface comes into contact with a protective gas before the mold is closed. It is possible for the protective gas to remove a volatile component from the surface of the monomer mixture over a shorter or longer period of time and at the very least, the mixture must be saturated to equilibrium with this component in the closed mold and its concentration at the surface reduced in this way.